In recent years, energy saving systems have been attracting interest in working machines such as hydraulic excavators and wheel loaders, and hybrid working machines and the like, which recover regeneration energy upon braking, have been put in the market. In many of such hybrid working machines which have been put in the market to date, however, their drive systems include an electrical system added to a conventional hydraulic system, and therefore are not different in that the flow rate to each hydraulic actuator is adjusted by controlling the opening of a control valve as a directional control valve, in other words, by restricting such a control valve while producing pressure loss.
For the energy saving of a working machine, the importance lies in the energy saving of its hydraulic system itself, and a significant effect can be obtained especially by reducing a restriction pressure loss that occurs across a control valve. As energy-saving driving device for working machines, developments are hence underway on closed hydraulic circuit systems that directly control each hydraulic actuator by connecting it to its corresponding hydraulic pump through a closed circuit. These systems use no control valve, and therefore are free of a pressure loss that would otherwise be produced by a control valve. Accordingly, the hydraulic pump delivers pressure oil at only a required flow rate, thereby making it possible to reduce a loss in flow rate. Further, these systems can regenerate the potential energy of such hydraulic actuators and the energy upon deceleration, and therefore are very effective systems as energy-saving systems.
With a closed hydraulic circuit system, however, the maximum output of a hydraulic actuator needs to be provided by a single hydraulic pump, leading to a problem that the pump becomes larger.
As a conventional technology that a closed hydraulic circuit system is configured without any increase in the size of a pump, there is the technology disclosed in Patent Document 1. According to the technology disclosed in this Patent Document 1, plural variable displacement hydraulic pumps are arranged, and the number of the pump(s) to be connected through a closed circuit to a hydraulic actuator and the delivery flow rate of each of the pump(s) are computed. By connecting each of the plural variable displacement hydraulic pumps to two or more hydraulic actuators through a closed circuit by way of selector solenoid valves and driving each hydraulic actuator with pressure oil from one or more of the hydraulic pumps, flow rates can be secured as desired by an operator without enlargement of the variable displacement hydraulic pumps.
In the case of a closed hydraulic circuit system, each variable displacement hydraulic pump is driven by an engine or electric motor that undergoes substantially uniform rotation, and the capacity of the variable displacement hydraulic pump is controlled by a regulator or the like to vary the delivery flow rate of the pump. In general, a variable displacement hydraulic pump has the characteristic that its efficiency is good in a large capacity range but is lowered in a small to medium capacity range. To further improve the energy saving effect of the closed hydraulic circuit system, it is desired to use each hydraulic pump in its large capacity range wherever possible.